Take it with a grain of salt, but my source is good.
After the patch 9.1 raid (Unknown, just that Kel'Thuzad turns evil and is the final boss) a cinematic where Kael'Thas Sunstrider, Jaina Proudmoore and Baroness Vashj speak to Kel'Thuzad's human form plays.
Kael'Thas asks Kel'Thuzad where they can find souls powerful enough aid them in their battle against the Jailer. Kel'Thuzad speaks of the soul chamber in Torghast. "Even... He... Is there." to which Kael'Thas turns to Jaina with a look of horror. The cinematic ends with Kel'Thuzad saying "His soul is the only one powerful enough to aid you." A close up of Jaina, who says "At what cost..." End of cinematic.
"He" obviously meaning Arthas.
Judging from the cinematic, the raid takes place in Maldraxxus. There is a lot of green-goo and such in the background. It's definitely not Naxxramas though.